1. Field of the Invention
The present invention relates to a fluorination catalyst and a process for fluorinating a halogenated hydrocarbon in a gas phase in the presence of the fluorination catalyst.
2. Description of the Related Art
Fluorinated halohydrocarbons such as 1,1,1,2-tetrafluoroethane are useful as substitutes for fluorocarbons and used as a refrigerant, a blowing agent, a propellant, a cleaning agent, and the like.
As a fluorination catalyst, chromium oxide which may be supported on alumina is known (see Japanese Patent Publication Nos. 10310/1964, 3004/1967 and 44973/1987, U.S. Pat. Nos. 3,426,009, 3,755,477 and 4,158,675 and GB 1 589 924). Also, fluorination in the presence of a chromium salt or partially fluorinated chromium oxide which may be supported on a carrier is known (see U.S. Pat. Nos. 2,745,886 and 2,885,427, DE Patent No. 1 252 182, Japanese Patent Publication No. 54503/1976, Japanese Patent Kokai Publication No. 132549/1978 and WO89/10341).
There are also known a catalyst comprising chromium oxide and an additive such as NaF (U.S. Pat. No. 3,644,545), Mg or Ba (Japanese Patent Publication No. 43922/1974), a transition metal (U.S. Pat. No. 4,792,643) or AlPO.sub.4 (Japanese Patent Publication No. 17413/1989). Further, there are known processes using a catalyst comprising metal chromium (Japanese Patent Kokai Publication Nos. 19038/1985 and 221338/1989) or a metal other than chromium (Japanese Patent Kokai Publication Nos. 186945/1987, 268651/1989, 172933/1990 and 95438/1990).
U.S. Pat. No. 4,766,259 discloses a fluorination reaction using partially fluorinated aluminum oxide.
A liquid phase fluorination reaction using a Sb catalyst is known. In addition, a liquid phase fluorination reaction using an alkali metal fluoride as a catalyst is known (see U.S. Pat. No. 4,311,863 and Japanese Patent Kokai Publication No. 228925/1989).
As the halogenated hydrocarbons, various compounds are used. The fluorination is explained by making reference to preparation of 1,1,1,2-tetrafluoroethane (hereinafter referred to as "134a") by the fluorination of trichloroethylene or 1,1,1-tetrafluorochloroethane (hereinafter referred to as "133a").
It is not advantageous to synthesize 134a from 133a by a liquid phase reaction in view of a low conversion and a material of a reactor. When this fluorination reaction is carried out in a gas phase, conversion of 133a to 134a is low due to equilibrium. Therefore, a catalyst to be used should catalyze this reaction at a relatively low conversion and have a sufficiently long life and a good selectivity in an industrial use. Prolongation of the catalyst life avoids frequent change of the catalyst and lowers the catalyst cost.
The catalyst life can be prolonged by the addition of chlorine gas (Japanese Patent Publication No. 33604/1977) or oxygen gas (GB Patent No. 2 030 981 and Japanese Patent Kokai Publication Nos. 82206/1976 and 272535/1989) to a reaction gas mixture. When the chlorine gas is added, selection of a material of a reactor may be limited and also increase in by-products will be considered. When the oxygen gas is added, a conversion may be decreased.
In view of the above, it is advantageous to provide a catalyst which has a long life as such. When such catalyst is excellent in catalytic activity, not only the catalyst cost but also a size of a reactor which is made of an high quality expensive material can be reduced advantageously.